With the increase in the availability of low voltage LED lights I'm investigating the cost and DIY benefits of switching the 12v side of the lighting rather than the 240v as is done at the SuperHouse. This would allow for a couple of powerpoints in the ceiling that driver high wattage (200W) power supplies to the feed relays that drive the LED lights. I've not conducted testing and am concerned about the efficiency of the power supply as it will always be on.

I like all of the other advantages of 12v switching as it can be wired and altered without an electrician and it can be distributed to an area of the house. Less masses of cable running to a couple of switchboards. I'm expecting the install costs to be lower due to the lack of large amount of time and cable to big switchboards.

I don't think this was really an option a few years ago and would like feedback on the positives and negatives of using this method.

The basic theory is to use a 8 port relay board plugged into and EtherTen to give individual control over 8 different lights. There could be 2-5 of these units in the roof to control all the lights in the house. Many lights are 240v wired now but as we will be replacing lots of them it would seems a good time to do something different. It would allow an incremental change in the lights and a single room could be moved from the current 240v setup to a 12v LED setup with a new Powerpoint switch. In the basic first iteration it would come out much like the superhouse where the lights are simply on/off using the switch. But I know it would not be much more programming to make lights to fancy on/off patterns and things like kill all the lights when you go to bed.

If you can use DC, remember that some PSU's (such as most ATX PC supplies) have remote power-on and a 'stand-by' 5V.

You could use the stand-by 5V to drive the Arduino side, and then have the Arduino switch the PSU on/off as needed. If you group the lights efficiently so that they're all in one room (or one section of a room) then you can intelligently switch off the PSU for that group when that section of the house isn't used. Things that aren't used very often (such as cupboards) could be grouped together so as to cut down usage. You will still have stand-by usage, but heaps better than what you'd get running the PSU's in 'on' mode.

Short answer: yes, that could be a good (and feasible) idea, with some caveats.

Running 12V lighting with more centralised power supplies is becoming far more practical now due to the decreased current requirements of LED fittings compared to other 12V fittings such as halogen globes.

The reason that halogen 12V downlights typically use little transformers (or switchmode supplies) fitted right near them is that the voltage drop on high-current runs across long cables is significant. It's the same reason that long-haul power lines run at thousands of volts: if cable resistance is fixed, and as Power = Current^2 x Resistance, there is more power loss as you decrease the voltage and increase the current.

So if you're running 12V systems it pays to have both the lowest current consumption possible, and also the shortest cable runs possible. To centralise the power supplies the cable run isn't a negotiable value, so you have to settle for running the lowest current (most efficient) lighting you can get. That's far easier now with LEDs, of course.

This also directly relates to Power-over-Ethernet schemes, and there's a section I've already filmed for Episode 3 (which will be all about PoE) that explains it and shows examples of power loss at 10V and 50V for a typical load at the end of a wire.
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Jon

In my houses design I could easily have 2-3 transformers with only 2-4 metres of 12V DC cable to connect up all the lights. The current design is lookings something like;

1 Arduino, 1 8xRelay board, 1x 100W LED driver and up to 8 LED lights connected. That allows individual LED switching which may or may not be individually controllable from light switches. 3 of those setups would create enough lights to cover all the lights we have now, plus 10 more.

There are dimmable power supplies available and I'm still investigating whether the dimmer can be controlled by hardware. I think it's a POT but haven't be able to verify. I'm not yet at the point of purchasing one.

PoE power drop is a reason I've opted for 20V PoE cheapy enabling the lowest current and the SwitchMode Power supply to bring it down to 7 volts. I just need a 12V SwitchMode supply to enable me to run 12V and 5V stuff.

My other outstanding concern about 12V switching has been the standby power loss of a 12V supply running about 80% effeciency and using 2-5Watts even when the power it off. Real 240V switching disables the power before it gets to the transformers so the power loss and effeciency aren't so much of an issue as it's on 5-8 hours a day, not 24.

mr-russ wrote:power supplies to the feed relays that drive the LED lights.

Personally, I'd be very tempted to choose an appropriate power semiconductor device like a MOSFET to switch the low-voltage DC to the LEDs on and off.

I don't like using relays unless there is no cost-effective semiconductor option that can handle the voltage and current involved. They're such a crude 19th century technology. They're bulky, noisy, relatively expensive, relatively power hungry, and they wear out.

Luke, you're right, relays are quite crude technology for this application. I'm using a bunch of relays (as you may have seen in one of the videos) and I only ever saw it as an interim solution. The major downsides for me are the lack of dimming control and the noise: they make quite a "CLACK" when they turn on. However, when the electrician was doing the cabling I found myself in a situation where I needed an immediate (and cheap!) solution to get it all going quickly, and settled on a combination of this relay:

The big hurdle with those is mounting. I don't know of any DIN rail bases for them, so I'll probably have to design a control board incorporating a bunch of them with screw terminals on one side and a logic interface on the other. I made a start on that about a year ago and haven't looked at it since, but it's about time I gave it more thought.

Just as a general comment about my approach to this project, I'm consciously not trying to come up with the perfect final solution for every part of the system the very first time. I'm working on the basis of doing something that gets me going as an immediate solution, then refining as I go based on lessons learned in practice. Obviously at each stage I do the best job possible, but only within the limits of what's practical to have a *working* system at that point in time. By "working", I mean a house I can live in!

One example is how I've done PoE. For the first iteration I started by cutting and splicing Ethernet cables by hand and soldering on DC jacks, which got me an instant (but ugly) solution for close to $0 cost.

Then for the second iteration I invested a bit more time and designed the 4-channel midspan injector (http://www.freetronics.com/poe-injector-4ch) which made things much neater and saved me time in the long run by reducing the manual cutting and splicing.

Then the third iteration (very recently) was when I found some PoE switches second-hand that I could afford, so now I'm progressively switching devices over to use 802.3af PoE (48V) instead. So my immediate problem was solved quickly and cheaply, then over time it's been made more robust.
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Jon

Interesting!
I am currently working on a similar project, except it will be in a new-built liveable extension (think granny flat). I've probably gone about it backwards as the building isn't even constructed yet, but I wanted to get a head start on the electronics so it could go in in a semi-functioning state right off the bat.

I'm planning on running all DC 12v LED for lighting in the whole place. (bearing in mind this is just a 9x6m 2 story 'shed' so distances won't be huge.)

I've got a Mega running all the code. It has 12 outputs (using 12 PWM-capable pins) driving a MOSFET for each output.

There is a further 12 outputs driving a relay for each of the outputs which will switch the +12v side of the circuit, while the MOSFET drives the GND. The code looks at the values of each of the lighting circuits; if it is > 0 it turns the relay on, if it's 0 it turns the relay off.
I implemented this part as a safety feature; using the MOSFET in the GND meant that even with the lights 'off', there was always +12v at one side of the light/load.
Whether that would be a problem or not remains to be seen, but the functionality is there if I decide to use it. I also thought it may be good if I end up using a non-dimmable load in one circuit, the relay can switch it and I'll just disable the MOSFET and make the code on or off rather than a PWM value. Relays are cheap so it was worth putting in.

It's also set up to monitor 12 inputs (switches) which will more than likely just be momentary push buttons on wall plates, wired back to the controller with Cat5 cable. Will send a GND in one of the cores so that the switches just pull that pin to the GND, with the inputs pulled high. This should eliminate any voltage drop issues over the Cat5.

The switches can either be for a discreet circuit on/off/dim. . (with the last circuit switched 'on' responding to the dim up/down requests) or 'scenes' which recall a value for all 12 circuits from EEPROM.
The code is there for both and it's just a matter of calling one or the other when the switch is pressed.

It also has IR remote control so any button on an IR remote can act as one of the switches, or do any other function such as saving the current 'look' to a scene memory in EEPROM.
Not quite sure how I will distribute the IR recievers yet. . most likely using a couple of cores of the Cat5 to the switches and fitting the IR receiver to the wall plate.

It's also running an RTC so time-based scheduling can do a 'wake-up' scene or outside lighting on and off based on time etc.

For power supply it will be centralised with a transformer to begin with, but the option is there to then add solar/battery power and just use the transformer if the batt voltage drops too low.

The coding and everything is all functioning now in a test bench scenario, IR remote, switches etc all work. .
I now just need to get the construction underway so I can get it happening for real!

I went through a similar decision process when building my house and decided that 240V switching is the best option.

The main deciding factor for me was that there is a great variety in power requirements for the various LED fixtures that may get used through a house which meant that just having 12V wouldn't work for all lights. In my construction, I ended up having some lights requiring 750mA (12-15V), some requiring 2000mA (~24V), some that were just 12V (as they were a serial circuit of stair lights).

In fact, 12V usually won't work well for any well made lighting fixture as the optimal way to drive an LED is with a constant current power supply to ensure consistent brightness, optimal efficiency (lumens/W), and to protect the actual LED from over-heating. This usually means a single dedicated driver per LED fixture.